Liposomes consist of at least one lipid bilayer membrane enclosing an aqueous internal compartment. Liposomes may be characterized by membrane type and by size. Small unilamellar vesicles (SUVs) have a single membrane and typically range between 0.02 and 0.05 μm in diameter; large unilamellar vesicles (LUVs) are typically larger than 0.05 μm. Oliglamellar large vesicles and multilamellar large vesicles have multiple, usually concentric, membrane layers and are typically larger than 0.1 μm. Liposomes with several nonconcentric membranes, i.e., several small vesicles contained within a larger vesicle, are termed multivesicular vesicles.
Conventional liposomes are formulated to carry therapeutic agents, drugs or other active agents either contained within the aqueous interior space (water soluble active agents) or partitioned into the lipid bilayer (water-insoluble active agents). Copending U.S. patent application Ser. No. 08/795,100 discloses liposomes containing cholesterol in the lipid bilayer membrane, where an active agent is aggregated with a lipid surfactant to form micelles and the micelles are entrapped in the interior space of the liposome.
Active agents that have short half-lives in the bloodstream are particularly suited to delivery via liposomes. Many anti-neoplastic agents, for example, are known to have a short half-life in the bloodstream such that their parenteral use is not feasible. These compounds also believed to distribute widely to many organs and tissues of the body to which they are toxic, thereby often limiting the concentrations that can be injected parentally. Encapsulation within liposomes typically helps to reduce this toxicity. Thus, the main goals of drug delivery are to retain drug in a biocompatible capsule thereby reducing toxicity, to avoid the body's defenses that normally recognize foreign particles and target them for removal by the liver and spleen, to instead allow targeting of the drug carrier to the therapeutic site of action, and once there, to release the drug rapidly so that it can act on the target tumor tissue. Conventional liposomes successfully achieve the first criterion, but, their use for site-specific delivery of active agents via the bloodstream is often limited by the rapid clearance of liposomes from the blood by cells of the reticuloendothelial system (RES). This problem was addressed by incorporating polyethyleneglycol lipids into the liposome membrane, that inhibits the protein adsorption that labels the liposome for RES uptake. Even if the liposomes can be made to accumulate at a diseased site such as a solid tumor, the drug is not necessarily released and available for efficacious activity; that ability to retain the drug often becomes an inhibitory factor at the tumor site.
Liposomes are normally not leaky but will become so if a hole occurs in the liposome membrane, if the membrane degrades or dissolves. Such a breakdown in permeability can be induced by the application of electric fields (electroporation), or exposure of the liposome to enzymes, or surfactants. Another, method involves raising the temperature of the membrane to temperatures in the vicinity of its gel to liquid crystalline phase transition temperature, where it appears that porous defects at phase boundary regions in the partially liquid and partially solid membrane allow the increased transport of water, ions and small molecules through the membrane. The clinical elevation of temperature in the body is called hyperthermia. This procedure has been used to raise the temperature at a target site in a subject and if temperature-sensitive liposomes can be delivered to the target site then this increase in temperature can cause the release of liposome contents, giving rise to the selective delivery of therapeutic agents, as initially described by Yatvin et al., Science 204: 188 (1979). This technique is limited, however, where the phase transition temperature of the liposome is significantly higher than the normal tissue temperature.
As an example, in order to begin to use this technology for the treatment of deep-seated tumors (e.g., prostate, ovarian, colorectal and breast tumors), it is accordingly desirable to devise liposome formulations capable of delivering therapeutic amounts of active agents in response to mild hyperthermic conditions, i.e., for clinically attainable temperatures in the range 39-41° C.